In batch coating of ferrous metals, such as in batch galvanizing, parts to be coated are immersed into a bath of coating material after having been chemically pretreated and cleaned. The amount of time the parts stay immersed depends upon the material of the parts, their shapes, the bath temperature, the coating composition, and the desired coating thickness.
Batch coating is frequently used to coat continuous strips of ferrous base metal to produce iron or steel strip stock having a thin coating of zinc, aluminum or the like. In continuous-strip batch coating, the strip to be coated is first cleaned and pretreated, passed through a bath of molten coating material, and then withdrawn from the bath in a generally upward direction. The coating material adhering to the withdrawn strip is finished by coating rolls, air knives, or the like, and is subsequently solidified.
The molten coating material, usually a molten metal such as zinc, for example, is contained in an externally-heated iron or steel pot. Metal coating pots have several disadvantages, however. The have a relatively short life. This is due to several factors: rapid build-up of dross on the bottom of the pot, creep or bulging of the pot walls caused by the high temperature of the external heat source, and forces on the pot walls caused by the weight of the molten coating metal in the pot.
With regard to pot life, a distinction must be made between pot durability (failure of the pot as a result of local reactions between molten coating metal and the iron or steel wall of the pot) and the pot utilization period (dissolution of pot material into the molten coating metal). A long pot life primarily depends on the throughput rate of strip to be coated and on the temperature of the inside pot wall, but also depends on the pot material.
The external heating system and its design also have a great effect on pot life. Heating systems in use today with iron or steel pots include gas and oil-fired systems, as well as electric heating systems (either resistance heating or induction heating). Uniformly distributed heat input over the overall heating surface of the pot is a precondition for maximum utilization of the calorific power of coating pots for metallic coatings. Thus, a long pot life requires that several, often competing, demands be met:
- careful and uniform heating PA1 - maintaining close temperature tolerances PA1 - if zinc is the coating metal, maintaining the inside pot wall temperature at or below 480.degree. C.
These demands are not always easily met in practice.
Another problem with batch coating is that the uncovered surface of the molten coating metal in the pot leads to the formation of oxides and dross at the surface of the molten coating metal. This is one of the most significant problems in hot dip batch coating. Upon emerging from the bath, the strip tends to pick up particles of dross and oxide from the surface of the bath, resulting in heavy edges of other imperfections in the coating applied.
One attempt to solve this last problem is disclosed in U.S. Pat. No. 3,887,721. That patent discloses a pot with a steel shell and a refractory lining, with an induction heating and stirring coil between the shell and the lining. The induction coil heats the molten metal bath and causes it to be continuously agitated so as to prevent dross accumulation on the bottom of the pot.
Although it does avoid the problems associated with conventional iron and steel coating pots, the solution proposed by U.S. Pat. No. 3,887,721 is less than ideal. By continuously stirring the molten metal bath, dross and oxides are kept in suspension and settle on the strip being passed through the bath, with the consequent undesirable effects on product quality.
The present invention provides a solution to the problem of how to avoid the problems associated with iron or steel coating pots while also avoiding the problems of dross and oxides being kept in suspension where they settle on the product and adversely affect its quality.